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Re: Why specify 3ksi concrete for grade beams?
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Why specify 3ksi concrete for grade beams?
- From: chuck utzman <chuckuc(--nospam--at)pacbell.net>
- Date: Sun, 10 Oct 2004 20:33:45 -0700
Hmm. In the hills around here a typical residential foundation is a
series of drilled piers connected together by "grade beams". AFAIK
nobody considers this to fall under the provisions of UBC
19188.8.131.52--because exception for "footings" the seems pretty clear. On
my drawings I require 3000 psi but state that the design is based on
2500 psi--no special inspection. I observe steel placement, call for
max. 3.5" slump, and provide the CBO with a letter at completion. Most
of the time the concrete used is 3/8" pea gravel & a 6 sack mix
(also spec'ed on my drawing) because it needs to be pumped.|
Aside from the cost of a special inspector & the test coupons, what
would you do with a completed foundation of 30 drilled piers & 100
cy of concrete with a framed structure on it, if the test lab said you
only had 2500 psi?
Chuck Utzman, P.E.
3000 psi concrete is also required
CBC 2001 section 19184.108.40.206 (and even more restrictive in section
which is the same as section 220.127.116.11 of ACI 318 2002
Thomas Hunt, S.E.
You guys had me up most of the night
early this morning as I could not recall why my specifications on my
demand f’c=3000 psi concrete for grade beams.
The best I could do was recall when we
first established RGA 1-91 back in the late 80’s. At the time, as I
it, all grade beams used in moment frames were required or strongly
to be designed using 3,000 psi concrete because it forced deputy
As I recall it (and I may be wrong but have used this spec since I
doing retrofit work in 1986) the city did not have a structural
requirement and wanted to force both the engineer of record and a
inspector to make sure the concrete exceeded 2,500 psi and that the
horizontal rebar as well as the shear ties (generally in place if
or not) were adequately placed.
It was a measure to force the community
to insure that a mix was adequate.
I had one instance where a small frame
was to be put in and the contractor decided to mix the concrete himself
on site. I objected to this after finding out what he had done. I
cores (it was in the city of Beverly Hills) and the stuff came out like
soup. The testing lab could not get a sufficient strength without the
simply falling apart to find out what strength it had.
The contractor complained, but we made
him remove all of the concrete he placed and had him do the job over
to the specifications that I provided for special deputy inspection. He
paid the price and the work was redone to the satisfaction of the
lab and the inspector.
I think that this practice originated
Los Angeles to force deputy inspection and verification of the strength
of the concrete when used to resist bending in the grade beam so as to
reduce the column size.
I’ve been the engineer of record on
of buildings in the Los Angeles county area from 1986 until 1993 and
of the grade beams I specified were done using f’c=3,000 psi concrete
and demanding special deputy inspection. It might have been unwritten,
I thought it was stated in RGA 1-91 or in the 1991 UCBC Appendix
5 but I can not find it so I must assume that it was a recommendation
the professional community at the time.
This is not to say that it was required
or that every engineer designed to this standard, but it was my
and since it exceeds the minimum requirements of the code and none of
clients complained, I believe it was the right choice to make.
Dennis S. Wish, PE
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